量子点发光器件的制备与仿真分析

为研究量子点发光器件结构与性能的关系,制备了以CdSe/ZnS量子点作为发光层、poly-TPD作为空穴传输层,Alq3作为电子传输层的量子点发光二极管,对器件结构及性能参数进行了表征,结果显示器件具有开启电压低、色纯度高等特点.结合测试数据,对量子点发光二极管进行了器件结构建模,利用隧穿模型及空间电荷限制电流模型对实验结果进行了分析,研究了器件中载流子的注入与传输机理.器件测试与仿真结果表明:各功能层厚度会影响载流子在量子点层的注入平衡,同时器件中载流子的注入与传输存在一转变电压,当外加电压低于转变电压时,器件中载流子的注入主要符合隧穿模型;当外加电压高于转变电压时,器件中载流子的注入主要符合空间电荷限制电流模型.研究结果验证了器件结构建模的合理性,可以利用仿真的方法进行器件结构优化并确定相关参数,这对器件性能的提高具有指导意义.     

Visfatin Mediates SCLC Cells Migration across Brain Endothelial Cells through Upregulation of CCL2

Small-cell lung cancer (SCLC) is characterized as an aggressive tumor with brain metastasis. Although preventing SCLC metastasis to the brain is immensely important for survival, the molecular mechanisms of SCLC cells penetrating the blood–brain barrier (BBB) are largely unknown. Recently, visfatin has been considered as a novel pro-inflammatory adipocytokine involved in various cancers. Herein, we present evidence that elevated levels of visfatin in the serum of SCLC patients were associated with brain metastasis, and visfain was increased in NCI-H446 cells, a SCLC cell line, during interacting with human brain microvascular endothelial cells (HBMEC). Using in vitro BBB model, we found that visfatin could promote NCI-H446 cells migration across HBMEC monolayer, while the effect was inhibited by knockdown of visfatin. Furthermore, our findings indicated that CC chemokine ligand 2 (CCL2) was involved in visfatin-mediated NCI-H446 cells transendothelial migtation. Results also showed that the upregulation of CCL2 in the co-culture system was reversed by blockade of visfatin. In particular, visfatin-induced CCL2 was attenuated by specific inhibitor of PI3K/Akt signaling in NCI-H446 cells. Taken together, we demonstrated that visfatin was a prospective target for SCLC metastasis to brain, and understanding the molecular mediators would lead to effective strategies for inhibition of SCLC brain metastasis.     

Forming-free bipolar resistive switching in nonstoichiometric ceria films

The mechanism of forming-free bipolar resistive switching in a Zr/CeO _x /Pt device was investigated. High-resolution transmission electron microscopy and energy-dispersive spectroscopy analysis indicated the formation of a ZrO _y layer at the Zr/CeO _x interface. X-ray diffraction studies of CeO _x films revealed that they consist of nano-polycrystals embedded in a disordered lattice. The observed resistive switching was suggested to be linked with the formation and rupture of conductive filaments constituted by oxygen vacancies in the CeO _x film and in the nonstoichiometric ZrO _y interfacial layer. X-ray photoelectron spectroscopy study confirmed the presence of oxygen vacancies in both of the said regions. In the low-resistance ON state, the electrical conduction was found to be of ohmic nature, while the high-resistance OFF state was governed by trap-controlled space charge-limited mechanism. The stable resistive switching behavior and long retention times with an acceptable resistance ratio enable the device for its application in future nonvolatile resistive random access memory (RRAM).     

Electrical properties of ITO/benzylated cyclodextrins (-CDs (Bz))/Al diode structures

Investigations of the electrical characteristics of benzylated cyclodextrins (β-CDs (Bz)) diodes are reported. We present current–voltage characteristics and impedance spectroscopy measurements performed on partially benzylated cyclodextrins β-CDs (Bz) thin films in sandwich structures ITO/β-CDs (Bz)/Al. The static electrical characterizations show a space charge limited conduction (SCLC) and a conductivity with power low frequency behavior characteristic of a hopping transport in disordered materials. The impedance spectra can be discussed in terms of an equivalent circuit model designed as a parallel resistor R_P and capacitor C_P network in series with resistor R_S. We extract numerical values of these parameters by fitting experimental data. Their evolution with bias voltages has shown that the SCLC mechanism is characterized by an exponential trap distribution. We estimated from the capacitance voltage characteristic an acceptor concentration of about due to trap states.     

非晶GaN薄膜的空间电荷限制电流特性

采用直流磁控溅射技术制备了Si衬底上的非晶GaN薄膜. GaN肖特基二极管伏安曲线不能简单地用包含串联电阻和复合电流效应的热电子发射理论来解释，其他电流输运机制（空间电荷限制电流）起主要作用. 分析数据得到平衡时的电子浓度为1.1E4cm-3和位于EC-0.363eV的陷阱能级. 测量空间电荷限制电流可以用来研究宽带隙化合物非晶半导体GaN的深能级性质.     

Effect of moisture (in solid polymer electrolyte) on the photosensitivity of conducting polypyrrole sensitized by prussian blue in solid-state photocells

The effect of small concentration of dispersed prussian blue (PB) [in solid polymer electrolyte viz polyvinyl alcohol (PVA) with phosphoric acid] on the photocurrents were studied (in wet state and in dry state of solid polymer electrolyte) in solid state photocells fabricated using conducting polypyrrole (PPy). These exhibit good photoresponse to visible light: photocurrents being 20 to 25 times greater than the dark currents depending upon the concentration of the sensitizer. The current–voltage (I–V) characteristics in such cells reveals that the charge transport is mainly governed by the space charge effects and the sensitization effects are due to the lowering of the potential barrier formed at the polypyrrole (PPy)/polyvinyl alcohol (PVA) interface under photoexcitation. Also, sensitization effects were studied in the wet state and in dry state of solid polymer electrolyte (PVA). It has been observed that the moisture content in the solid polymer electrolyte reduces the photosensitivity factor (S=I_l/I_d, where I_l is the light current and I_d is the dark current), since it increases the ionic conductivity in the solid polymer electrolyte thereby increasing the dark current values which results in overall decrease in the photosensitivity factor (S). The importance of the other factors such as dye concentration, dye aggregation etc. have been explained on the basis of the experimental observations. The sensitization effects are explained on the basis of the energy band diagram of the materials forming the photocells.     

Nanoscale imaging of dense fiber morphology and local electrical response in conductive regioregular poly(3-hexylthiophene)

We present a thermal annealing method for producing micrometer long and dense semi-crystalline fiber structures in poly(3-hexylthiophene) (P3HT) films. The thin film topology and local electrical properties are investigated by conductive atomic force microscopy (c-AFM), and three-dimensional device models are used to illustrate the effects of current spreading, anisotropic mobility and traps. In-plane current flow along high mobility fibers provides a mechanism for c-AFM contrast in fibrous films. The comparison of c-AFM results between the annealed and non-annealed P3HT films suggests that trap effects are pronounced in disordered P3HT thin films prior to thermal annealing. The methodologies we demonstrate here on the archetypal P3HT film can be generalized for understanding the correlation between structure and local electrical properties in a variety of polymer and polymer nanocomposite systems.     

Space-charge and injection limited current in organic diodes: A unified model

Space-charge and injection limited currents in organic diodes have been analyzed using a unified model. Both currents have been modeled using the transport equations combined with a proper value of the free charge density at the metal–organic interface. The method has been applied to diodes with different organic materials, metal contacts and lengths of the organic materials. This unified model accurately reproduces published current–voltage curves for a variety of diode structures operating at different temperatures and voltages. The results of the method, in the injection regime, have been compared with the results of pure injection models developed by other authors. The reduction of the computational time and the number of parameters in our model are important advantages of our procedure. Moreover, it is an alternative where the injection models start to fail: at low applied voltages, close to the Ohmic regime, and for low heights of the energy barrier at the interface. The treatment is complemented with a compact model that relates the current density j with the free charge-carrier density at the interface p_f(0): p_f(0) = K₁j^m + K₂, where the parameter K₁ depends on the barrier height at the interface, m depends on the organic material and K₂ controls the flat zone at low currents to include the dependence with thermal carriers and impurities.     

Influence of electrode on electrical conduction of ZincPhthalocyanine (ZnPc) thin films

The electrical conduction properties of ZincPhthalocyanine (ZnPc) thin films have been studied using copper, silver and aluminium electrodes. The sandwich structures were prepared by the thermal evaporation method. The I–V characteristics were investigated to identify the dominant charge transport mechanism in the films. Among all possible mechanisms, it was observed that the data fits well to the SCLC type of conduction in the Al/ZnPc/Al and Schottky type of conduction prevails in the Ag/ZnPc/Ag and Cu/ZnPc/Cu devices. The trap levels and its dependence of structure have been studied and results are discussed. The charge transport phenomenon in the ZnPc films seems to depend highly on the electrode material and temperature. The carrier mobility increases with increasing temperature whereas the density of trapped holes decreases with increasing temperature. The barrier height also decreases with increase in temperature. The influence of the temperature on the electrical parameters such as saturation current density (J_s), barrier height (Φ_b), density of states in the valence band edge N_d (m⁻³), the position of the Fermi level E_F (eV), ionized acceptor atom density N_e (m⁻³), activation energy ΔΦ (eV), mobilities of hole (μ₀) and the concentration of free holes in the valence band (n₀) have been discussed in detail.     

Space‐charge limited conduction in polyaniline films

Polyaniline emeraldine base (PANI‐EB) powder was synthesized by oxidative polymerization of aniline. The PANI‐EB films were prepared by the solution‐casting technique. The temperature‐dependent dc conductivity measured in the range 173–303 K suggests that the PANI‐EB is a quasi‐one‐dimensional disordered conductor. The current‐voltage characteristics of the PANI‐EB films measured in the range 333–383 K showed the SCLC mechanism. The SCLC parameters such as free carrier density (p₀), trap density (p_t), the ratio between free carrier density to the total carrier density (θ), mobility (µ) and the effective hole mobility (µ_eff) were calculated. The activation energy (E_a = 0.32 eV) and the Fermi level (E_F = 0.42 eV) were estimated. As well as these, the trap parameters such as the trap filled limit voltage (V_TFL), the shallow trap density (N_t), the depth of the dominant trap level (E_t ? E_v), the density of states within the hole mobility edge (N_v) and the characteristic energy (E_c) were also calculated and presented. The exponential type of traps distribution with large number of traps was found to be due to the disorder and moisture in the polymer films. Copyright © 2004 Society of Chemical Industry